(1) Field of the Invention
The present invention relates to an electrophotographic photosensitive material comprising an m-phenylenediamine compound as a charge transport substance.
(2) Description of the Related Art
An organic photosensitive material which has a good processability, is advantageous from the viewpoint of the manufacturing cost and has a large freedom of design of functions is recently used as an electrophotographic photosensitive material in an imageforming apparatus such as a copying machine. Especially, photosensitive layers comprising a charge generating material capable of generating charges under irradiation with light and a charge transport material for transporting the generated charges, for example, function-separated single-layer type photosensitive materials comprising a charge generating material, a charge transport material and a binder resin, and function-separated electrophotographic photosensitive materials having a laminate type photosensitive layer having a laminate structure comprising a charge generating layer containing a charge generating material as described above and a charge transport layer containing a charge transport material as mentioned above, have been proposed.
The Carlson process is widely used for forming a copied image by using an electrophotographic photosensitive material. The Carlson process comprises, in principle, the charging step of uniformly charging a photosensitive material by corona discharge, the light-exposure step of exposing the charged photosensitive material imagewise to light to form an electrostatic latent image corresponding to an original image, the developing step of developing the electrostatic latent image with a developer containing a toner to form a toner image, the transfer step of transferring the toner image to a substrate such as a paper sheet, the fixing step of fixing the toner image transferred onto the substrate, and the cleaning step of removing the toner left on the photosensitive material after the transfer step. In order to form a high-quality image in this Carlson process, it is required that the electrophotographic photosensitive material should be excellent in charging characteristics and photosensitive characteristics, and the residual potential after the light exposure should be low.
In the above-mentioned function-separated electrophotographic photosensitive material, the electric characteristics and photosensitive characteristics of the photosensitive material are greatly influenced by the characteristics of the charge generating material and charge transport material. Accordingly, various substances have been examined, and polyvinyl carbazoles, oxadiazole compounds, pyrazoline compounds and hydrazone compounds have been proposed as the charge transport material.
However, in these charge transport materials, the drift mobility indicating the charge transport capacity is relatively small. Moreover, the dependency of the drift mobility on the electric field intensity is large, and therefore, the migration of charges in a low electric field is small and cancellation of the residual potential is difficult. Moreover, these charge transport materials are readily deteriorated under irradiation with ultraviolet rays and the like.
It is known that the electric field dependency of the drift mobility is small in the triphenylamine type charge transport material. For example, U.S. Pat. No. 3,265,496 proposes N,N,N',N'-tetraphenylbenzidine, N,N,N',N'-tetraphenyl-1,4-phenylenediamine and N,N,N',N'-tetraphenyl-1,3-phenylenediamine. However, these charge transport materials are still defective in that the sensitizing effect is low in the ordinary application and if the amount added is increased, crystallization is often caused. Therefore, these charge transport materials cannot be put into practical use.
For overcoming this defect, we previously proposed m-phenylenediamine compounds, in which phenyl groups of N,N,N',N'-tetraphenyl-1,3-phenylenediamine can optionally be substituted, so far as the substitution is possible (see Japanese Patent Application No. 62-301703).
Certain improvements can be attained by introduction of substituents, but it has been found that the improvements are still insufficient.
In general, the characteristics of an electrophotographic photosensitive material are greatly influenced by the photoconductive material used, though the influences differ more or less according to the object and preparation process.